1. Field of the Invention
The invention relates to the use of polypropylene glycol as a shrinkage-reducing additive in dispersion powder compositions for construction materials, in particular dry mortars, which contain Portland cement as an inorganic binder.
2. Background Art
Dispersion powders are not new per se and have been used for many years particularly in the construction industry as a synthetic resin modifier of hydraulically setting systems. An overview of the effect of dispersion powders in construction materials is given in the journal Tonindustrie TIZ 9, 1985, p. 698. Significant improvements in the properties of adhesion, abrasion resistance, scratch resistance and tensile strength in bending are obtained by the addition.
Dispersion powders are prepared by spray-drying aqueous polymer dispersions with the addition of polyvinyl alcohol and other additives. The readily pourable powder with particle sizes between 10 and 250 .mu.m is redispersed in water to form the original dispersion again, with particle sizes between 0.1 and 5 .mu.m. The redispersions should be stable even over a relatively long period, that is to say they should not tend to sediment.
Such dispersion powders are described, for example, in EP-A 228 657, DE-OS 26 14 261 or DE-PS 20 49 114.
EP-A 407,889 describes dispersion powders based on water-soluble salts of phenolsulfonic acid-formaldehyde condensation products as an atomization colloid.
The great advantage of dispersion powders compared with the liquid dispersions which can also alternatively be used is that dry mortar mixtures can be produced which only have to be mixed with water on the site. This has many advantages such as increased work safety, reliable handling or simple disposal of drums.
One of the negative properties of cement, in particular the Portland cement mostly used, is the shrinkage or contraction of the hydraulic materials, which leads to volume changes of the setting mortars or concretes. If these volume changes become too large, this can lead to cracking in the construction materials. Efforts are therefore made to keep the shrinkage tendency as low as possible.
Standard dispersion powders, consisting of the components of dispersion, atomization colloid, polyvinyl alcohol or phenolsulfonic acid-formaldehyde condensation products, relatively small quantities of additives and antiblocking agents, such as in EP-A 228 657 or EP-A 407 889, lead in construction materials which contain Portland cement as inorganic binder to an increased shrinkage of the material.
Although the shrinkage can be reduced via the formulation by using shrinkage preventing agents, since dry mortars are to be produced pulverulent materials are in general required. A spraying process, which is possible in principle, is laborious and expensive. DE-A 37 32 114 describes pulverulent materials based on alkane diols with terminal OH groups. These materials have the disadvantage that they are hygroscopic and usually not odor-neutral.
JP-A-73 43 014 (C.A. 79: 96272v) proposes adding secondary or tertiary alcohols, specifically glycerine, to cement-containing materials to prevent the shrinkage. The disadvantage in using these compounds as cement additives, however, is the very severe delaying of the setting of the hydraulic materials because of the hygroscopic behavior of these compounds; in addition these products are liquid.
JP-A-59 128 240 (C.A. 101: 176465y) describes polyoxyethylene ethers with a terminal OH group as shrinkage-reducing additives in mixtures with fluorine-containing emulsifiers. The disadvantage is that these are liquid additives to be added separately to the dry mortar mixture.
EP-A2 116 524 claims the use of gypsum for reducing the shrinkage. The disadvantage of this procedure is that, when using gypsum in combination with Portland cement, there is always the danger of expansion due to the gypsum, with the entire strength of the construction material being lost (Czernin, Zementchemie fur Bauingenieure, Bauverlag).
Polyalkylene glycols have hitherto been added in small quantities (German Offenlegungsschrift 40 30 638: 0.1 to 2% by weight) to dispersion powder compositions as foaming inhibitors.